1. Field of the Invention
The invention relates to a discharge lamp of the short arc type which is used, for example, as a light source for a projection apparatus and is filled with xenon, or discharge lamp of the short arc type which is used as a light source in semiconductor exposure and is filled with mercury.
2. Description of the Related Art
In a discharge lamp of the short arc type which is filled, for example, with xenon, there are cases in which the breakdown voltage is greater than or equal to 30 kV, by which the voltage charge of the operating device is significantly raised. Here, there is the disadvantage that the starting voltage of the operating device punctures and the disadvantages of increasing the size of the operating device and increasing costs. Proceeding from such a circumstance, there is a trigger wire located on the outside surface of the arc tube and thus the breakdown voltage of the lamp is reduced.
FIG. 4 is a schematic of a conventional discharge lamp of the short arc type in which the outside surface of the arc tube is provided with a trigger wire. In the figure, an arc tube 11 contains a pair of electrodes, specifically an anode 21 and a cathode 22. The outside surface of the arc tube 11 is provided with a trigger wire 5. The trigger wire 5 is formed with an annular area 51 which is in contact with the outside surface of the arc tube 11. Furthermore, there is a holding part 52 which is connected to this annular area 51 and which extends in the lengthwise direction of the lamp. Part of this holding part 52 is wound around a side tube part 12 which adjoins the arc tube and is attached and is connected to the base 42 on one end of the holding part 52.
FIG. 5 shows a perspective view of only the trigger wire 5. Here, the holding part 52 is connected by welding or the like to a part of the annular area 51 which is formed in the shape of an uninterrupted ring. The annular area 51 has elasticity to the extent to which it is deformed by application of a force. The length of the wire-like component comprising the annular area 51 is however always constant and never changes. The diameter d of the annular area 51, in the state before its installation on the arc tube 11, is smaller than the maximum diameter D of the arc tube shown in FIG. 4. By inserting the annular area 51 from the direction of the base 41 into the lamp the annular area 51 comes into contact with the outside surface of the arc tube 11, is slipped on and attached.
FIG. 6 is a schematic of a conventional discharge lamp of the short arc type in which the trigger wire is wound differently. The arc tube 11 is provided with the remainder of the filler tip 1a. The trigger wire 5 is provided with an attachment part 50 which is attached by one end being wound around the remainder of the filler tip 1a. A holding part 52 which extends in the lengthwise direction of the lamp is connected to this attachment part 50. Part of this holding part 52 is wound around the side tube part 12 which is connected to the arc tube 11, and attached. One end of the holding part 52 is connected to the base 42.
The lamp shown in FIG. 7 has a base 41 that is provided with an installation part 41a of a diameter Dxe2x80x2 that is greater than the maximum diameter D of the arc tube 11 and which is used for mounting the lamp is mounted in a device. In such a case in which the diameter Dxe2x80x2 of this installation part 41a is greater than the maximum diameter D of the arc tube 11, there is the disadvantage that the trigger wire 5 which is shown in FIG. 4, having the uninterrupted annular area 51, cannot be inserted onto the lamp from the direction of the base 41, because the diameter of the annular area 51 of the trigger wire 5 is smaller than the maximum outside diameter of the arc tube 11, and thus, smaller than the diameter of the installation part 41a. 
In this case, as is shown in FIG. 8, the annular area 51 is inserted onto the lamp from the base 42 on the opposite side, the annular area 51 is located on the outside surface of the arc tube 11 and one end of the holding part 52 is connected to the base 41.
In the operating process, in which a high voltage is applied to the cathode 22 which is located on the side opposite the anode 21 and which is not electrically connected to the trigger wire 5, the trigger wire 5 being electrically connected to the anode 21 via the base 41, the high voltage is not applied to the trigger wire 5. The trigger function therefore does not take effect here. In a lamp in which such an operating process is undertaken, therefore, there is inevitably the disadvantage that the trigger wire cannot be in the optimum state.
The lamp shown in FIG. 6 has the remainder of a filler tip 1a for attachment of the trigger wire 5. However, in a lamp which does not have a remainder of a filler tip 1a, there is the disadvantage that the winding process shown in FIG. 6 cannot be used to wind the trigger onto the arc tube.
Furthermore, in an arrangement of the trigger wire as is shown in FIG. 5, there is also a process in which the shape of the trigger wire does not have a given shape beforehand, but in which the wire-like component which is to become the trigger wire is bent on the outside surface of the arc tube and installed. This case is described below using FIG. 4. By bending this wire-like component into a ring shape, an annular area 51 is formed. Afterwards, using a tool, such as pliers or the like, one end of the annular area 51 is bent around. The wire-like component is turned such that the annular area 51 is formed uninterruptedly in alignment and that the length of the wire-like component comprising the annular area 51 thereafter never changes. A holding part 52 can also be made in alignment on this turned area.
When such a process is used in which a wire-like component which is to become the trigger wire is bent and installed on the surface of the arc tube, it becomes possible to arrange the trigger wire on the arc tube with optimum directional capacity, regardless of what kind of electrode, whether anode or cathode, to which the high voltage is applied. However, it is necessary to carry out the work of bending or twisting the wire-like component on the surface of the arc tube into an annular shape. This poses the disadvantage that the tool can come into contact with the surface of the arc tube and the surface of the arc tube is scratched as a result.
In the case of the trigger wire 5 which is shown in FIG. 6, there is also the disadvantage that, when the trigger wire 5 is attached in the remainder of the filler tip 1a, the tool comes into contact with the surface of the arc tube 11 and the surface 11 of the arc tube is scratched.
The invention was devised to eliminate the above described defects in the prior art. Thus, a primary object of the present invention is to devise a discharge lamp of the short arc type in which the following is enabled:
There can be a trigger wire even in a lamp which has a base with a diameter which is larger than the maximum outside diameter of the arc tube;
The trigger wire can be arranged in the state which is appropriate to the alignment of the electrode to which a high voltage is applied during operation;
The trigger wire is located on the arc tube at an optimum site, even if the arc tube does not have any remainder of the filler tip; and
The arc tube is not scratched when the trigger wire is installed on it.
The object is achieved in the invention described, according to the invention, in a discharge lamp of the short arc type in which a pair of electrodes are located within an arc tube and in which the outside surface of this arc tube is provided with a trigger wire, in that the above described trigger wire has:
an annular area which is elastically deformed when the trigger wire is installed on the outside surface of the arc tube and which is looped around the arc tube and attached;
a first holding part adjoins one end of the annular area and extends in the axial direction of the arc tube;
a second holding part adjoins the other end of the annular area and extends in the axial direction of the arc tube.
The invention is described below using drawings.